Severe multi-address interference and inter-symbol interference exist in CDMA mobile communication systems. In a slot CDMA system, a joint detection method can be used to eliminate the multi-address interference and inter-symbol interference for a received local cell signal. The joint detection method takes a signal detection process as a unified inter-related joint detection process by using transmission signals and channel response information of all users in the local cell. Since the joint detection technology effectively restrains the inter-symbol interference and the multi-address interference in the local cell, performance of the CDMA system is improved greatly.
A time division-synchronous code division multi-address (TD-SCDMA) system is a kind of slot CDMA system adopting the joint detection solution for multi-user signal in the local cell. In a TD-SCDMA system, a structure of a burst signal in a normal slot is shown in FIG. 1, wherein a midamble (also called channel estimation code) in the middle of the burst signal is used for channel estimation and the data block at the two sides of the burst signal are used for service data transmission.
A detailed process of a single cell joint detection in the TD-SCDMA system is as follows:
Firstly a signal model received by a single cell receiver in a slot CDMA system is obtained.e=Ad+n  (1)
In the above formula (1), e represents sampled data of a signal received by the receiver, d represents transmitted data, n represents a received noise and matrix A is a transmission matrix. The transmission matrix A is composed of a combined response vector b(k) of each code channel, wherein assuming there are K code channels all together and k represents a serial number of the code channel (the matrix A can be computed according to b(k) and b(k) is a column in each block distributed on a diagonal of the matrix.).
Each vector b(k) corresponds to a combined channel response of a user code channel whose serial number is k.b(k)=C(k)⊕h(k) k=1 . . . K  (2)
In the above formula (2), C(k) is a spreading code of the user code channel whose serial number is k, ⊕ is a convolution operator and h(k) is a channel response of the user code channel whose serial number is k and h(k) is obtained by performing channel estimation with the midamble.
Then the joint detection can be performed with the above information. There are many kinds of joint detection algorithm. The joint detection algorithm can be an interference cancellation method or a block linear equalizer method or a combination of the two methods. For the block linear equalizer method, a soft output value estimated by demodulated transmitted data d is:{circumflex over (d)}=(T)−1A*TRn−1e  (3)
The matrix T is obtained from the following formula:
                    T        =                  {                                                    I                                            MF                                                                                                          A                                          *                      T                                                        ⁢                                      R                    n                                          -                      1                                                        ⁢                  A                                                                              ZF                  -                  BLE                                                                                                                                                A                                              *                        T                                                              ⁢                                          R                      n                                              -                        1                                                              ⁢                    A                                    +                                      R                    d                                          -                      1                                                                                                                    MMSE                  -                  BLE                                                                                        (        4        )            
In the above formula, *T is a conjugation transposition; Rd=E{d·d*T} is a covariance matrix of a data sequence d; for independent data sequence, Rd=I; and Rn=E{n·n*T} is a covariance matrix of the noise sequence n. For independent and stable noise sequence (such as white noise), Rn=σ2I, wherein I represents a unit matrix.
In the case of Rd=I and Rn=σ2I, formulae (3) and (4) can be simplified as:
                              d          ^                =                                            (              T              )                                      -              1                                ⁢                      A                          *              T                                ⁢          e                                    (        5        )                                T        =                  {                                                    I                                            MF                                                                                                          A                                          *                      T                                                        ⁢                  A                                                                              ZF                  -                  BLE                                                                                                                                                A                                              *                        T                                                              ⁢                    A                                    +                                                            σ                      2                                        ⁢                    I                                                                                                MMSE                  -                  BLE                                                                                        (        6        )            
The MF in formulae (4) and (6) is matching-filter corresponding to a matching-filter method; ZF-BLE is a Zero-Forcing Block Linear Equalizer method corresponding to a maximum likelihood linear solution; MMSE-BLE is a Minimum Mean Square Error Block Linear Equalizer method corresponding to a minimum mean square linear solution. Any one of the above three methods for obtaining T can be used and generally the ZF-BLE or the MMSE-BLE method is selected.
The single cell joint detection method uses the structure information of signals (including the spreading code and the channel response) transmitted by all the users in the cell, i.e. uses the structure information of signals in the local cell. At this time, the detection method processes the interference signals from other cells as the Gauss white noise of the time, therefore, the single cell joint detection method is more effective in restraining the inter-symbol interference and multi-address interference in the local cell. The single cell joint detection method can greatly improve the system performance for a single cell or for a CDMA system adopting inter-frequency networking.
For a slot CDMA system adopting same frequency network for neighboring cells, the single cell joint detection method can also improve the system performance. However, in the case of the same frequency network, there is strong interference between signals from intra-frequency neighboring cells. The interference between signals from intra-frequency neighboring cells impacts the system performance greatly. Especially at the border place of intra-frequency neighboring cells, the intra-frequency interference is generally the most important interference. At this time, the single cell joint detection method cannot restrain the intra-frequency neighboring cell interference so when the intra-frequency neighboring cell interference exists, the system performance will decrease greatly.
For example, when users, at the border place of cells, of intra-frequency neighboring cells work on several code channels, especially when the system is one with a small spreading gain, the signals of the intra-frequency neighboring cell users will highly interfere the signal of the local cell. In this case, the single cell joint detection method will deteriorate the system performance extremely.
Thus, when the slot CDMA cellular mobile communication system works in intra-frequency neighboring cells, the single cell joint detection method will cause the deterioration of the system performance, so there is a need for providing a better joint detection method that can enable better system performance. If the joint detection method can be extended from a single cell to intra-frequency multiple cells by using the structure information of the intra-frequency neighboring cells, the performance of the slot CDMA system in same frequency network will be improved greatly.
In order to obtain the structure information of the intra-frequency neighboring cells except that of the local cell, much apriori information is needed, wherein a kind of important apriori information is the channel estimation result of the intra-frequency neighboring cells except the local cell, i.e. multi-cell channel estimation result. Since when generating the channel estimation code, each neighboring cell takes a different midamble as a basic code and different users in the cell use cyclic shift versions of this basic code as their channel estimation codes. Thus, the different cyclic shift versions of the basic code of one cell can be called a code set, multiple cells have multiple code sets and the channel estimation of the multiple code sets is that of the multiple cells.
In the Chinese patent application “a multiple code set channel estimation method for a slot CDMA system” with the application No. 03100670.1, a multi-cell estimation method for a slot CDMA system is disclosed.
The above mentioned technical solution performs finite time decision processing for the single code set channel estimation result and only keeps finite taps with stronger signal responses so as to recover interference signals of other code sets, to cancel the interference signals, to obtain net signals of every code set, and to perform the single code set channel estimation for the net signal of each code set. The above processing is repeated for iterative times and then multiple code set (multi-cell) channel estimation result is outputted.